Ich meinte eher:
Get-Content c:\test.txt | ForEach-Object {($_ -replace "sAMAccountName","Kuerzel"); ($_ -replace "sn","name")} | Set-Content c:\test1.txt
Alternativ:
$_.replace("Wert1_Old","Wert1_New").replace("Wert2_Old","Wert2_New").replace("Wert3_Old","Wert3_New")